A stem cell (SC) is characterized by two properties: 1) the unique capacity for self-renewal (a stem cell is able to divide into identical progeny cells indefinitely, perhaps throughout the entire life of the organism), and 2) in response to a signal such as a growth factor, to give rise to a cell containing an altered pattern of gene expression and a more restricted developmental potential than its parent. Eventually, a SC becomes known as a “progenitor” or “precursor” cell, committed to producing one or a few terminally differentiated cells such as neurons or muscle cells (Fischback, et al., 2004, J. of Clin. Invest., 114:1364-1370). Thus, in contrast to the large majority of cells in the body committed to a specific function, SCs are uncommitted and remain so until they receive a signal to generate specialized cells.
A totipotent cell is a stem cell not committed to a specific lineage, which is capable of giving rise to all types of differentiated cells and tissues, including extraembryonic tissues. The only type of totipotent stem cell is the fertilized egg. Soon after fertilization, in the earliest stages of embryogenesis, a zygote is formed by the fusion of the egg and sperm, and the zygote and its progeny divide several times to form a ball of 32 to 128 cells called a “morula.”. Each cell of the morula is totipotent in that each one can give rise to all cell types in the embryo plus all of the extraembryonic tissues necessary for implantation in the uterine wall. As cells of the morula continue to proliferate, the morula enlarges to form a hollow sphere called a blastocyst. Next, but still prior to implantation, a few cells delaminate from the surface layer of the blastocyst to form an inner cell mass (ICM) within the cavity. Blastocysts created in vitro also contain an ICM, and it is possible to isolate cells from the ICM of human blastocysts and grow them in tissue culture. Cells isolated from the ICM are pluripotent, as they can become any of the hundreds of cell types in the adult body, but they are not totipotent because they cannot contribute to extraembryonic membranes or the formation of the placenta (Fischback, et al., 2004, J. of Clin. Invest., 114:1364-1370). Thus, a single pluripotent SC has the ability to give rise to cells originating from all three germ layers: mesoderm, endoderm, and ectoderm. The only known sources of human pluripotent SCs are those isolated and cultured from the inner cell mass of the blastocyst, known as embryonic stem cells (ESCs), and those isolated from the primordial germ cells of the gonadal ridge of 5- to 10-week fetuses or embryonic germ cells (EGCs) (Lemoli, et al., 2005, Haematologica, 90:360-81).
In undifferentiated human ESCs, several marker genes have been shown to be expressed and may play a role in pluripotent capacity. These include, but are not limited to, the POU-domain transcription factor OCT4 (Hansis, et al., 2000, Mol. Hum. Reprod., 6: 999-1004; Niwa, et al., 2000, Nat. Genet., 24, 372-376); Growth and Differentiation Factor 3 (GDF3), Nanog, stella-related (STELLAR) gene (Clark, et al., 2004, Stem Cells, 22: 169-179); Pumilio-2 (PUM2) (Moore, et al., 2003, Proc. Natl. Acad. Sci. USA, 100: 538-543) and Nanos 1 (Jaruzelska, et al., 2003, Dev. Genes Evol., 213: 120-126; Clark, et al., 2004, Human Mol. Genet., 13: 727-739; Cavaleri, et al., 2003, Cell, 113: 551-557).
SCs have also been identified in nonembryonic tissues. Adult SCs are undifferentiated cells present in various differentiated mature tissues. Like ESCs, adult SCs are undifferentiated, but are considered to be multipotent, having the potential to give rise to a more limited number of cell types. Adult SCs are capable of differentiation into the cell types from the tissue that the adult stem cell originated. Furthermore, in the past decade, adult SCs have been found in tissues that were not previously believed to harbor them, such as the central nervous system. Adult SCs have been derived from the nervous system (McKay, 1997, Science 276:66-71; Shihabuddin, et al., 1999, Mol. Med. Today 5:474-480), bone marrow (Pittenger, et al., 1999, Science 284:143-147; Pittenger, et al., 2001, in: Mesenchymal stem cells of human adult bone marrow. Marshak, D. R., Gardner, D. K., and Gottlieb, D. eds., Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 349-374); adipose tissue (Gronthos, et al., 2001, J. Cell. Physiol. 189:54-63), dermis (Toma, et al., 2001, Nature Cell Biol. 3:778-784) and pancreas and liver (Deutsch, et al., 2001, Development 128:871-881) and other organs (Lemoli, et al., 2005, Haematologica, 90:360-81).
Notably, SC derived from adult tissues such as the bone marrow (BM), the skeletal musculature, or the central nervous system (CNS) have been recently reported to have a more plasticity in differentiation potential than was originally believed to be possible for adult SCs. In the last few years, a number of different groups have claimed that adult mammalian SCs may be capable of differentiating across tissue lineage boundaries and that this capacity to transdifferentiate into mature cells of different origin may represent a novel therapeutic strategy for tissue regeneration. (Lemoli, et al., 2005, Haematologica, 90:360-81). For example, a subset of adult mesenchymal stem cells (MSCs) derived from bone marrow have been reported to be pluripotent (Jiang, et al., 2002, Nature, 418:41-9). Additionally, a subpopulation of stem cells within adult human BM were reported to self-renew without loss of multipotency for more than 140 population doublings and exhibit the capacity for differentiation into cells of all 3 germ layers. These multipotent stem cells from human BM were reported to regenerate myocardium after myocardial infarction (Yoon, et al., 2005, J. Clin. Invest., 115: 326-338).
Adult stem cells are the focus of intensive research aimed at developing transplantation strategies to promote recovery in the diseased or injured tissues. Adult stem cells are maintained in the niche microenvironment. Transdifferentiation of an adult SC into a non-canonical progeny, e.g., muscle or liver from BM SC, has been a rare phenomenon, usually associated with severe damage in the target tissue, and often with a specific selective pressure for the trans-differentiated progeny. Furthermore, some of the reports were not confirmed in subsequent investigations; for instance, the muscle-derived SC reported to give rise to hematopoietic SC upon transplantation were subsequently shown to be hematopoietic in origin. In other cases, cell fusion rather than transdifferentiation was demonstrated to be the main mechanism of the observed plasticity of adult SC. Nevertheless, the hope of finding pluripotency in adult SCs has an obvious relevance for regenerative medicine. The possibility of using SC from easily accessible sources to repair or regenerate tissues severely damaged by diseases such as muscular dystrophy, diabetes mellitus, Alzheimer's, or hepatitis, or by vascular conditions, autoimmune disorders, congenital and/or degenerative disorders, disease or trauma, such as infarcted myocardium, cirrhotic liver, connective tissue damaged by rheumatic disease, would have a dramatic therapeutic impact on otherwise untreatable conditions (Lemoli, et al., 2005, Haematologica, 90:360-81) as well as on basic research, drug discovery, treatment and prevention of disease.
Hair follicles have a well defined epithelial stem cell niche: the bulge, which is a well-demarcated structure within the lower permanent portion of hair follicles. Bulge epithelial stem cells have limited differential potential and have been shown to give rise to only squamous and sebaceous cells. Hair color is determined by melanocytes in the hair bulb at the base of hair follicles. While melanocyte precursor cells were postulated in the bulge area as shown in melanocyte-targeted (Dct)-lacZ transgenic mice, these cells have not be isolated and it is unknown whether they are present in human hair follicles.
Thus, there remains a great need for methods of isolating new sources of pluripotent SCs, especially those bearing the genetic hallmarks of early embryonic SCs.